Use of soluble Ziegler-Natta type catalyst systems in the polymerization of olefins, in particular polymerization of ethylene to polyethylene is known in the art. In general, traditional Ziegler-Natta type systems comprise a transition metal halide activated to form a catalyst species by reaction with a metal alkyl cocatalyst, particularly aluminum alkyl cocatalysts. However, aluminum alkyl cocatalysts are often used in large excess, see U.S. Pat. No. 4,404,344. This is disadvantageous because the aluminum compounds must be removed from the resultant polymers. These traditional Ziegler-Natta catalysts often contain a variety of different active sites, each of which has its own rate of initiation, propagation, and termination. As a consequence of this non-uniformity of active sites, the linear polyethylene has a broad molecular weight distribution. See for example, Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, 1982; Vol. 3, Chapter 22.5, p 475; Transition Metals and Organornetallics as Catalysts for Olefin Polymerization; Kaminsky, W. and Sinn, H., Eds.; Springer-Verlag: New York, 1988, and Transition Metal Catalyzed Polymerizations: Alkenes and Dienes; Quirk, R. P., Ed.; Harwood: New York 1983.
Recently, catalysts have been reported that rely on boron rather than on aluminum-containing compounds. Boron-based catalysts, in contrast, to the aluminum-based catalysts are often stoichiometric in boron. That is, they contain one mole of boron-containing component per mole of transition metal. Furthermore, it is usually unnecessary to remove the small amount of boron from the polymer, unlike the aluminum-based catalysts, mentioned above.
Tris(pentafluorophenyl)borane (C.sub.6 F.sub.5).sub.3 B forms 1:1 complexes with Lewis bases such as ether, amines, and phosphines. The compound is hygroscopic and, presumably forms a monohydrate but neither the composition, that is stoichiometry of this hydrate nor its properties have been disclosed. No uses for these donor-acceptor complexes have been taught, see Massey et at. J. Organomet. Chem. 1964, 2, 245. Hygroscopic (C.sub.6 F.sub.5).sub.3 B.Et.sub.2 O was reported by Pohlman et al. Z. Nat. 1965, 20b, 5.
Hlatky et al. J. Am. Chem. Soc. 1989, 111, 2728 described zwitterionic catalysts such as (Me.sub.5 Cp).sub.2 Zr[(m--C.sub.6 H.sub.4)BPh.sub.3 ]. EPO 0 277 004 describes catalysts prepared by reacting, (Me.sub.5 Cp).sub.2 ZrMe.sub.2 with B.sub.9 C.sub.2 H.sub.13, [Bu.sub.3 NH][(B.sub.9 C.sub.2 H.sub.11).sub.2 Co] or [Bu.sub.3 NH][B.sub.9 C.sub.2 H.sub.12 ]. Alternatively, ammonium salts of B(C.sub.6 F.sub.5).sub.4 -- may be used, WO 91 02,012; Chem. Abstr. 1991, 114, 247980u.
Similarly, EPO 0 418044 describes monocyclopentadienyl complex catalysts containing a non-coordinating, compatible anion such as (C.sub.6 F.sub.5).sub.4 B--. More recently, homogeneous catalysts exemplified by [Cp.sub.2 ZrMe][MeB(C.sub.6 F.sub.5).sub.3 ] have been synthesized from the reaction of Cp.sub.2 ZrMe.sub.2 and (C.sub.6 F.sub.5).sub.3 B see X. Yang et at. J. Am. Chem. Soc'y 1991, 113, 3623.
Furthermore, the above described catalysts are sparingly soluble in toluene. The catalysts are even less soluble in normally liquid .alpha.-olefins such as 1-hexene or in mixtures of such olefins and non-reactive solvents, such as hexane, toluene or xylene. These catalysts generally separate as oils from toluene or toluene-hexane mixtures. Even though catalysis still proceeds, phase separation is undesirable for several reasons, for example contact between monomer and catalyst is less efficient when the catalyst is only partially soluble. When the catalyst is incompletely soluble, catalyzed polymerization typically takes place at different rates either in solution or at the solid-liquid interface, thus tending to lead to a broad distribution of polymer molecular weights. Furthermore, catalyst:monomer ratio in solution is generally difficult to control when the catalyst is only partially soluble.
It is further known that a soluble or molecularly dispersed catalyst typically permits more ready access of the substrate to the active sites. As a result, more efficient use of the catalyst is possible. It is also recognized that the molecular weight of a polymer is proportional to the concentration of monomer in the reaction mixture in which it is synthesized. Generally, high molecular weight is desirable in applications such as glues and adhesives, as well as in the construction of rigid objects such as gaskets, insulators and packaging materials.
Catalytic polymerization of lower olefins, in particular ethylene and propylene is relatively easy. On the other hand polymerization of longer chain .alpha.-olefins tends to be slower and the products are often oligomers rather than high polymers, see Skupinska Chem. Rev. 1991, 91, 635. Heterogeneous catalysts such as TiCl.sub.3 /AlEt.sub.3, which produce higher molecular weight polymers from long-chain .alpha.-olefins, lead to a broad range of molecular weights (high polydispersity index).